1. Field of the Invention:
The present invention relates to tourniquets, especially to tourniquets used to provide a bloodless field in surgery.
2. Description of the Prior Art:
Tourniquets have been used when performing surgery on the arms and legs to provide a bloodless field in the area of the operation and therefore greatly simplify the procedure. In the past, the use of excessive pressure to close off the blood vessels created a condition where injuries of several types, including limb paralysis and nerve damage, occurred. To avoid the deleterious effects of using a tourniquet, it was therefore desirable to use as low a tourniquet pressure as possible to stop blood flow to the limb. It was not practical or effective to use medically trained personnel to constantly monitor and adjust tourniquet pressure to maintain the bloodless field yet avoid tissue or nerve damage. To simplify the operation and reduce costs of the operation a system as automatic and error free as possible was needed so that personnel need not be assigned to monitor the tourniquet. The prior art moved in that direction but did resolve all the problems.
One prior vital signs monitor used a variation of the oscillometric method to measure blood pressure and heart rate. An inflatable blood pressure cuff was placed on the test subject and the cuff was inflated to a preset pressure. The monitor then used the incremental deflation technique to determine systolic, diastolic and mean arterial blood pressure. This device sought two equal value pressure pulses at each given cuff pressure, then deflated the cuff by one pressure increment. It then looked again for two equal value pressure pulses. If the pulses at the first several cuff pressures were not of near equal value, the device increased the pressure in the cuff and performed this check again. When it found several successive cuff pressures that showed equal value blood pressure pulses, it would begin its measuring sequence. It would deflate the cuff by the predetermined pressure increment and measure the value of the blood pressure pulses. When the blood pressure pulses started increasing in value, the device determined that this was the systolic blood pressure. When the pressure pulse value had been increasing and began decreasing, it determined that this was the mean arterial pressure. When the pulses stopped decreasing in value and leveled off, this was determined to be the diastolic blood pressure. This is the oscillometric technique coupled with incremental deflation to determine blood pressure. This device then displayed the determined blood pressures and heart rate.
While this variation of the oscillometric method did provide one method of determining blood pressure parameters, it was not reliable or accurate. These matters were, of course, significant since the patient's well being was being concerned. The prior art oscillometric method had a tendency to produce erroneous blood pressure readings. The blood pressure pulses were sufficiently variable, and the sampling and error checking limited enough, so that aberrant blood pressure results occured and it was necessary to repeat the test.
U.S. Pat. No. 4,321,929 disclosed an automatic tourniquet. This device relied upon sensing Korotkoff sounds, i.e., characteristic sounds made by blood flowing under pressure from the heart, to determine blood pressure. An inflatable cuff was placed over the subject. The cuff was then inflated until blood flow ceased, and the cuff was then gradually bled down to atmospheric pressure. When Korotkoff sounds were detected above a certain preset level, the pressure in the cuff was deemed to correspond to the systolic pressure. When the device stopped sensing Korotkoff sounds, the cuff pressure was equated to the diastolic pressure. The device then adjusted tourniquet pressure according to some unspecified relationship with the determined blood pressure.
U.S. Pat. No. 3,552,383 and English Pat. No. 1,253,501 also showed a blood pressure monitor. This device also used an inflatable blood pressure cuff and detected Korotkoff sounds to determine blood pressure values. This device tested for Korotkoff sounds at two different thresholds. If the sounds went from below level one through a middle region and above level two, the pressure in the cuff was equated to the systolic region and the systolic pressure was correlated to the cuff pressure where the sounds commenced. If the sounds went from above the higher level two through a middle region to below the lower level one, the pressure in the cuff was equated to the diastolic region and the diastolic pressure was determined to be the cuff pressure when the sounds stopped or went below the lower level. The device could increment or decrement the pressure in the cuff and did not rely on deflation-only operation. Even though this device sampled many heart beats at a given cuff pressure and required the Korotkoff sounds to be within a certain time after an EKG pulse corresponding to a heart beat its reliability and accuracy were insufficient to satisfy modern surgical needs.
Both this device and the previous device relied on sensing Korotkoff sounds. There were inherent problems with this approach. Korotkoff sounds were highly variable. They varied on a given person under different conditions and varied dramatically between individuals. This made any automatic approach very difficult and prone to errors. Simple devices, like the two discussed, were prone to errors and variability.
U.S. Pat. No. 4,469,099 disclosed a pneumatic tourniquet that used a pressure transducer and microprocessor to accurately maintain the desired, preset tourniquet pressure. The inventor had described the device in Complications of and Improvements in Pneumatic Tourniquets used in Surgery, Medical Instrumentation, July 1981, at 352. In the article he suggested that it might be possible to include an automatic sphygomomanometer with the tourniquet. He followed up on his suggestion and was issued in U.S. Pat. No. 4,479,494. This patent disclosed the use of a blood pressure sensor in combination with a controllable tourniquet to produce an adaptive pneumatic tourniquet. The patent did not disclose any new technique for sensing the blood pressure, but merely suggested the use of commonly available blood pressure monitors, one of which has been previously discussed. In this regard, see also an article entitled An Adaptive Tourniquet for Improved Safety in Surgery, IEEE Transactions on Biomedical Engineering, Vol. BME-29 No. 2, Feb. 1982, at 122.
The major problem with the prior art was a tendency to produce aberrant blood pressure readings with some frequency. If these aberrant readings were then used as a basis for an automatic control device to control tourniquet pressure, without human monitoring and intervention, problems resulted because the tourniquet responded to erroneous signals and did not operate as needed. With systems of the prior art, resolution of this aberrant reading problem required more time than was available and made the devices highly unresponsive to the changing conditions of the patient in the operating environment.